Estimates of the generation of scrap tires produced in the United States are on the order of 2 million tons per year. Although these tires contain a high percentage of useful hydrocarbons, high grade steel and carbon black, approximately 70% are disposed of in landfill sites, open dumps, or stockpiled. This results in serious environmental problems as well as the loss of potential economic value. Because tires do not degrade, they are not good candidates for landfills, and open dumping may result in accidental fires which produce high pollution emissions. Other common fates of scrap tires include shredding for combustion or as an additive to road asphalt. While these approaches provide a simple method of waste reduction, air emission concerns are inherent and optimal recovery of the energy and chemical components are not obtained.
Recently, pyrolytic recycling of scrap tire, i.e. thermal decomposition in the absence of O.sub.2, is receiving renewed interest because of its ability to produce hydrocarbon oils that can be used as fuel additives. These hydrocarbons typically have high energy content on the order of 40-50 MJ/kg (1700 BTU/lb). In addition, this process also permits the recovery of the high grade steel typically used in tires as well as carbon in the form of smokeless fuel, carbon black, or activated charcoal.
A recent evaluation of pyrolytic recycling of scrap tires indicates that it can be an economically viable process if marketable products can be efficiently collected. Due to the economic and environmental attraction, a number of benchscale and pilot studies for tire pyrolysis have been recently reported. These systems typically use inert atmospheres and decompose the rubber at temperatures ranging from 700.degree.-900.degree. C. Under these conditions, the maximum collectable yield of the hydrocarbon product is only 30% of the tire mass. However, when the temperature is decreased to 500.degree.-700.degree. C., the yield of hydrocarbon oil is increased to 40-50% of the tire mass. Although the reduction of temperature permits both a savings in the process energy required and an increase in the amount of condensible hydrocarbons, low temperature pyrolysis also produces a viscous mixture of high molecular weight carbon compounds (C.sub.10 -C.sub.20) having a high C:H ratio (e.g. creosote and polycyclic aromatic hydrocarbons), also known as soot. Unfortunately, these compounds reduce the effectiveness of the pyrolysis resulting in poor thermal degradation of the rubber and making product reclamation difficult, if not impossible. In addition, the difficulty in removing these hydrocarbons often leads to the build up of combustible materials in pyrolysis ovens. Another drawback to these compounds is that they are not easily recyclable.
One material of interest that is produced in high quantities during the pyrolytic decomposition of tires is limonene. Limonene has many extremely fast growing industrial applications. It is used in the formulation of industrial solvents, resins, adhesives and as a dispersing agent for pigments. It is also used as a feed stock for the production of fragrances and flavorings. Limonene is biodegradable, a natural solvent, environmentally safe with excellent solvency, rinsability and high wetting penetration and detergent properties. It has been used in a wide range of applications including water-based degreasers, natural lemon-scented all-purpose cleaners, hand cleaners and replacements for chlorofluorocarbon solvents to clean electronic circuit boards.